1. Field of the Invention
The present invention is a device for quantitating afferent optic nerve defects which produce a Marcus-Gunn pupil.
2. Description of the Prior Art
Every ophthalmologist is familiar with the swinging flashlight test, where an ordinary light is first shown in one eye, then the other, then back again. In the normal patient, one observes a direct pupilary constriction in both eyes, as the light strikes each one in succession. In a patient with an afferent defect, however, the consensual constriction will be greater than the direct such that, when the light is moved towards the afferently-deficient eye from the intact eye, the former eye's direct response will be so much less than its consensual that its pupil will appear to dilate in response to the flashlight. This abnormality is termed the Marcus-Gunn effect and is a major diagnostic sign of afferent optic nerve defects.
At present, there are several methods for quantitatively measuring Marcus-Gunn phenomena. One method is to place a series of neutral density filters in front of the intact eye, and to repeat the swinging flashlight test. More particularly, this method is performed by rapidly increasing the density of the filters in front of the intact eye until the defective eye's consensual reflex is observed to be reduced to the same level as its impaired direct reflex. When this endpoint is reached, the eye with the afferent defect will show no dilation when the light illuminates it; i.e., its consensual reflex has been reduced to equal its direct.
Unfortunately, this method suffers from several disadvantages. For example, retinal bleaching poses a significant problem; since no filter is ever placed over the afferently-defective eye, that eye will gradually constrict more as more room light is constantly illuminating it. Hence, the observer quickly notes that, as the test proceeds, he must gradually increase the amount of absorption used in front of the intact eye to achieve the same endpoint. Clearly, retinal bleaching, which occurs in a matter of fractions of seconds, makes this method's reproducibility highly dependant on the dexterity of the physician.